This invention relates to a process for preparing water swellable sorptive composite products which is adaptable to conventional paper making equipment.
It is well known that a variety of different polymers and polymer forming systems can modify and enhance the water sorptive properties of substances such as cellulose. Such water swellable polymers and systems employing such polymers are illustrated by U.S. Pat. Nos. 3,670,731; 3,954,721; 3,959,569; 3,980,663; 3,989,586; 4,041,020; 4,041,228; 4,041,231. The methods available for employing any of these polymer systems in preparing water sorptive products are controlled in large part by the properties which these systems possess at the time they are utilized.
Typically, the water swellable polymers disclosed in the above mentioned patents possess acid groups which are converted by reaction with base to the salt form before the water sorbing properties are imparted thereto. Thus, in those instances where the salt form of the polymer is produced during polymer preparation or immediately thereafter such polymers cannot be economically employed as additives to cellulose pulp slurries in a paper making process using conventional paper making equipment, e.g. a Fourdrinier, cylinder machine or Rotoformer. This results from the fact that use of a polymer in its highly swellable salt form ensures that it will likely sorb many times its weight in water. Consequently, a substantial fraction of the total water in the paper furnish or stock will be bound in the polymer which is incorporated therein and the wet sheet coming off the Fourdrinier wire or press section can contain as much as 100 pounds of water per pound of polymer. Drying of such a sheet would be prohibitively expensive. By comparison conventional paper manufacture involves evaporation of only about 2 to about 4 pounds of water per pound of paper. For a process to be economical the amount of water to be evaporated should be as close as possible to this range and should be no more than about 10 times the weight of the paper.
The problem of moisture control in a paper making process is implicitly recognized in U.S. Pat. No. 3,989,586 and is solved by postponing the conversion of the potentially water swellable copolymer disclosed therein, i.e. a crosslinked, water insoluble copolymer of maleic anhydride and a vinyl monomer, until after the paper sheet containing said copolymer has been dried. Even then, the neutralization is carried out with a substantially anhydrous alkaline agent such as ammonia gas or an alcoholic solution containing a base. While the use of an anhydrous alkaline agent prevents undue sorption of water prior to drying the paper sheet, such an approach is accompanied by its own set of problems. For example, ammonia gas is very toxic and special provisions must be made for its containment. Since conventional paper making machines are typically unsealed, the use of ammonia gas is commercially unfeasable. Furthermore, the use of alcohol as a solvent for the base is very expensive relative to basic aqueous solutions.
It would therefore be a distinct advantage if a method could be developed for making water sorptive products which is capable of preventing undue moisture sorption during neutralization of the water swellable polymer with an aqueous alkaline solution. This would allow one to employ conventional paper making equipment and improve the cost efficiency of the process.
However, in addition to the problems described above, the development of such a method is further complicated by the requirement that when the potentially water swellable polymer is advantageously applied as a solid to the cellulose fibers it must adhere strongly thereto during the paper making process and must remain in the solid state even after neutralization. When neutralization is achieved with an aqueous alkaline solution, many potentially water swellable polymers, while insoluble in their acid form, completely dissolve when converted to their salt form due to the sorption of water.
For example, it has been noted by Verbrugge in an article entitled "Mechanism of Alkali Thickening of Acid-Containing Emulsion Polymers" reported in the Journal of Applied Polymer Science, Vol. 14, pages 897 to 928, 1970, that acid containing latexes during neutralization all undergo a common swelling process leading to complete solubilization for the more hydrophilic polymers. Verbrugge described specific neutralization reactions in a series of polymers of varying Tg and hydrophilicity of the general formula methyl methacrylate/ethyl acrylate/methacrylic acid (MMA/EA/MAA) utilizing 20 mole % of MAA and varying ratios of MMA/EA from 50/0 to 0/80. Observations were made of both viscosity changes and visual changes in a light microscope. Work is also disclosed at the higher hydrophilic range of monomer, e.g., utilizing high EA/MAA contents such as 80/20 and 70/30 EA/MAA with zero MMA. The light microscope shows gradual swelling of the particles as the carboxylic acid groups were progressively neutralized. At 80% neutralization, the particles are disclosed as being so highly swollen as to make the phase change from solid to liquid medium barely distinguishable. At 90% and 100% neutralization, the particles are gone and a true solution is formed. Polymer latex particles which are less hydrophilic, e.g., those observed from high MMA levels and lower EA and MAA, e.g., 60/20/20 MMA/EA/MAA, result in viscous gels upon neutralization and are seen in the light microscope as barely discernable swollen gel particles with no clear boundary between the swollen particle and the solution. Electron microscope examination of the dried 100% neutralized latex particle shows numerous tentacles projecting from the central core which result in association or sticking together of particles which in turn result in high viscosity.
When solid particles of potentially water soluble polymers are deposited on cellulose fibers during a paper making process and then subsequently dissolved upon neutralization the paper tends to stick to the rollers which increases the chances that the sheet will break. Furthermore soluble polymers will initially form highly viscous gelled regions within the paper which will seal the paper pores and inhibit further water sorption. Upon still further contact with water the soluble polymer will be leached from the paper sheet causing contamination of the surrounding regions.
Thus, the search has continued for an economic process for making water sorptive paper products wherein conventional paper making equipment can be employed, said paper products relying on the presence of water swellable polymers for their sorptive properties. The present invention is a result of this search.